linux-kernel - Re: [PATCH 1/3] perf/core: Make sure the ring-buffer is mapped in all page-tables

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Message-Id: <D89602E9-E620-4AF0-822C-206D7F0BA071@amacapital.net>
Date:   Fri, 20 Jul 2018 12:20:24 -1000
From:   Andy Lutomirski <luto@...capital.net>
To:     Joerg Roedel <jroedel@...e.de>
Cc:     Andy Lutomirski <luto@...nel.org>, Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Juergen Gross <jgross@...e.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>, Jiri Kosina <jkosina@...e.cz>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Brian Gerst <brgerst@...il.com>,
        David Laight <David.Laight@...lab.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Eduardo Valentin <eduval@...zon.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Will Deacon <will.deacon@....com>,
        "Liguori, Anthony" <aliguori@...zon.com>,
        Daniel Gruss <daniel.gruss@...k.tugraz.at>,
        Hugh Dickins <hughd@...gle.com>,
        Kees Cook <keescook@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Waiman Long <llong@...hat.com>, Pavel Machek <pavel@....cz>,
        "David H . Gutteridge" <dhgutteridge@...patico.ca>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>
Subject: Re: [PATCH 1/3] perf/core: Make sure the ring-buffer is mapped in all page-tables


> On Jul 20, 2018, at 11:37 AM, Joerg Roedel <jroedel@...e.de> wrote:
> 
>> On Fri, Jul 20, 2018 at 12:32:10PM -0700, Andy Lutomirski wrote:
>> I'm just reading your changelog, and you said the PMDs are no longer
>> shared between the page tables.  So this presumably means that
>> vmalloc_fault() no longer actually works correctly on PTI systems.  I
>> didn't read the code to figure out *why* it doesn't work, but throwing
>> random vmalloc_sync_all() calls around is wrong.
> 
> Hmm, so the whole point of vmalloc_fault() fault is to sync changes from
> swapper_pg_dir to process page-tables when the relevant parts of the
> kernel page-table are not shared, no?
> 
> That is also the reason we don't see this on 64 bit, because there these
> parts *are* shared.
> 
> So with that reasoning vmalloc_fault() works as designed, except that
> a warning is issued when it's happens in the NMI path. That warning comes
> from
> 
>    ebc8827f75954 x86: Barf when vmalloc and kmemcheck faults happen in NMI
> 
> which went into 2.6.37 and was added because the NMI handler were not
> nesting-safe back then. Reason probably was that the handler on 64 bit
> has to use an IST stack and a nested NMI would overwrite the stack of
> the upper handler.  We don't have this problem on 32 bit as a nested NMI
> will not do another stack-switch there.
> 

Thanks for digging!  The problem was presumably that vmalloc_fault() will IRET and re-enable NMIs on the way out.  But we’ve supported page faults on user memory in NMI handlers on 32-bit and 64-bit for quite a while, and it’s fine now.

I would remove the warning, re-test, and revert the other patch.

The one case we can’t handle in vmalloc_fault() is a fault on a stack access. I don’t expect this to be a problem for PTI. It was a problem for CONFIG_VMAP_STACK, though.

> I am not sure about 64 bit, but there is a lot of assembly magic to make
> NMIs nesting-safe, so I guess the problem should be gone there too.
> 
> 
> Regards,
> 
>    Joerg